Update C status page rationale for Annex F conformance

This is setting expectations that Clang will likely never conform to
Annex F on 32-bit x86 with SSE2 disabled because of the way x87 works.

clang/www/c_status.html

@@ -306,7 +306,10 @@ <h2 id="c99">C99 implementation status</h2>
           glibc</a>, <a href="https://git.musl-libc.org/cgit/musl/tree/include/stdc-predef.h">
           musl</a> will define the macro regardless of compiler support unless
           the compiler defines <code>__GCC_IEC_559</code>, which Clang does not
-          currently define.
+          currently define.<br>
+          Additionally, Clang intentionally will not conform to Annex F on
+          32-bit x86 without SSE2 due to the behavior of floating-point
+          operations in x87.
         </details>
       </td>
     </tr>